PSA: Vitamin D deficiency is a strong factor, not for getting Covid-19, but for bad outcomes. It's especially common for people with dark skin, since that blocks the ability of producing Vitamin D from sunlight. If that's part of what's happening to the Navajo, I don't know, but you should probably eat Vitamin D pills, especially during this indoor lockdown, and if if you have dark skin.
The Navajo have been in the general area (the US forcibly moved them, but the latitudes are similar) of where they are now for a long time. Their skin tone should be well-adapted to the region, unless they're not going outside.